An axial flow, gas turbine engine has a compression section, a combustion section and a turbine section. An annular flow path for working medium gases extends axially through the sections. A stator assembly extends about the annular flow path for confining the working medium gases to the flow path and for directing the gases along the flow path.
As the gases are flowed along the flow path, the gases are pressurized in the compression section and burned with fuel in the combustion section to add energy to the gases. The hot, pressurized gases are expanded through the turbine section to produce work. A major portion of this work is used for useful purposes, such as driving a free turbine or developing thrust for an aircraft.
A remaining portion of the work generated by the turbine section is not used for these purposes. Instead it is used to compress the working medium gases. A rotor assembly extends between the turbine section and the compression section to transfer this work from the turbine section to the compression section. The rotor assembly in the turbine section has rotor blades which extend outwardly across the working medium flow path. The rotor blades have airfoils which are angled with respect to the approaching flow to receive work from the gases and to drive the rotor assembly about the axis of rotation.
An outer air seal circumscribes the rotor blades to confine the working medium gases to the flow path. The outer air seal is part of the stator structure and is formed of a plurality of arcuate segments. The stator assembly further includes an outer case and a structure for supporting the segments of the outer air seal from the outer case. The outer case and the support structure position the seal segments in close proximity to the blades to block the leakage of the gases past the tips of the blades. As a result, the segments are in intimate contact with the hot working medium gases, receive heat from the gases and are cooled to keep the temperature of the segments within acceptable limits.
An initial radial clearance is provided between the seal segments and the tips of the rotor blades to avoid destructive interference between these parts during operation of the engine. The clearance is needed because the outer air seal, the outer case, and the rotor blades move radially at different rates in response to changes in temperature of the hot working medium gases.
The size of the radial clearance depends on the operative conditions of the engine and varies during operation of the engine. To minimize this clearance at cruise or other steady-state operating conditions of the engine, cooling air is discharged against the outer case to cause the case to contract. The contracting case displaces the seal segments inwardly to a smaller diameter and decreases the clearance between the rotor blade tips and the outer air seal with a beneficial effect on engine efficiency.
Examples of such constructions are shown in U.S. Pat. No. 4,019,320 issued to Redinger et al. entitled "Clearance Control For Gas Turbine Engine" and U.S. Pat. No. 4,337,016 issued to Chaplin entitled "Dual Wall Seal Means".
As can be seen in these patents, each seal segment is spaced circumferentially from the adjacent segments leaving a clearance gap G for each pair of segments between the sides of the segments. The clearance gap G for each pair of segments has an initial value G.sub.max. The initial value G.sub.max compensates for tolerance variations, such as variations in segment length caused by manufacturing tolerances, so that as the outer case contracts and forces the outer air seal to a smaller diameter, destructive contact between the sides of segments does not occur. The smallest minimum clearance value G.sub.min occurs at the operating condition of the engine which forces the sides of the segments closest together and will likely occur between those pairs of segments having the greatest circumferential length and the smallest inital value G.sub.max.
As mentioned earlier, the seal segments are cooled to maintain the temperature of the segments within acceptable limits during operation of the engine. In Chaplin, a primary flow path for this cooling air is in flow communication with the seal segments. The outer case, which has passages for the primary flow path, provides an outer boundary for the flow path. A seal means, such as an impingement plate, extends between the working medium flow path and the primary flow path for cooling air to provide an inner boundary to the primary flow path. The impingement plate is spaced from each segment leaving a cavity therebetween. Secondary flow paths, such as a secondary flow path extending through the cavity, direct cooling air to each outer air seal. A plurality of first holes extend through the impingement plate to place the primary flow path in flow communication with the secondary flow path. The first holes precisely meter the flow of cooling air to the secondary flow path. A plurality of second holes extend through each outer air seal segment from the cavity to the radially extending side of one of the segments which bounds the clearance gap G. The holes place the clearance gap G in flow communication with the secondary flow path.
Cooling air is flowed through the primary flow path, the first holes, the secondary flow path in the cavity, and the second holes in the seal segment to the circumferential gap G. The cooling air is at a pressure greater than the pressure of the adjacent working medium flow path to ensure that cooling air flows into the flow path and that working medium gases do not flow into the holes in the seal segments. The size of each second hole determines the flow rate of cooling air through the hole into the gap G for a given operative condition of the engine. Typically, an empirical method is used to determine the hole size. The method includes the step of increasing the size of the holes in each segment until all seal segments are sufficiently cooled during operation of an experimental engine. As a result of tolerance variations, some segments are over cooled in production engines to ensure that all segments in the engine are sufficiently cooled.
The use of cooling air increases the service life of the outer air seal in comparison to uncooled outer air seals. However, the use of cooling air decreases the operating efficiency of the engine because a portion of the engine's useful work is used to pressurize the cooling air in the compressor. A decrease in the amount of cooling air required to provide a satisfactory service life for components such as the outer air seal increases the work available for other purposes, such as providing thrust or powering a free turbine, and increases the overall engine efficiency.
Accordingly, scientists and engineers are seeking to more efficiently supply cooling air to components such as outer air seal segments and to minimize the overcooling of such components.